The overall goal of our research is to understand the molecular mechanisms of toxicity/carcinogenicity of environmental pollutants and the endogenous cellular defense systems to cope with pollutants. Drinking water contaminated with arsenic, a known carcinogen, is a worldwide public health issue. Epidemiology studies have linked arsenic exposure to human cancers, including skin, liver, lung, kidney, prostate, and bladder cancer. Arsenic can also cause cellular damage through generation of reactive oxygen species (ROS) that are even involved in the initiation, promotion, and progression of tumors. Although arsenic is a well defined carcinogen, it is not mutagenic and induces malignant transformation possibly by an epigenetic or cell signaling mechanism. Eukaryotic cells have evolved several defense mechanisms to cope with stress from the environment, one of which is the antioxidant response utilized by mammalian cells to neutralize ROS and to maintain cellular redox homeostasis. This antioxidant system is mediated through the antioxidant response element (ARE) sequence present in the promoters of several antioxidant and Phase II detoxification genes including glutathione S-transferase, NAD(P)H quinone oxidoreductase, glutamylcysteine synthetase, and heme-oxygenase. The antioxidant response system is mainly controlled by the transcription factor Nrf2. Activated by compounds possessing anti-cancer properties, the ARE-Nrf2-Keap1 signaling pathway has been clearly demonstrated to have profound effects on tumorigenesis. More significantly, Nrf2 knockout mice display increased sensitivity to chemical toxicants and carcinogens and are refractory to the protective actions of chemopreventive compounds. Therefore, we hypothesize that activation of the ARE-Nrf2-Keap1 pathway acts as an endogenous protective system against arsenic-induced toxicity and carcinogenicity. The following specific aims are intended to further elucidate the mechanism of Nrf2-activation in protection from arsenic-induced toxicity/tumorigenicity. This knowledge can potentially serve the scientific and medical community in our objective to create novel chemopreventive agents with increased specificity and efficacy, which will have broad impact on human health worldwide. We propose to (1) determine the protective role of the ARE-Nrf2- Keap1 pathway in arsenic-induced toxicity and carcinogenicity, (2) define the molecular mechanisms of activation of the ARE-Nrf2-Keap1 pathway by arsenic, and (3) define the protective role of the ARE-Nrf2-Keap1 pathway in arsenic-induced toxicity and tumorigenicity using the Nrf2 knockout mouse model.